What is half-nut mechanism?

Answer 1

A half nut is literally a nut split in half, length wise and makes up the basic half-nut mechanism.

Such a mechanism is typically found on a engine lathe.

This mechanism can be "open" or "closed" and when closed clamps onto the lead screw of said lathe.

This forms a connection between the apron of the lathe sliding on the ways. The apron carries the cross and compound slides, the latter in turn holds the cutting tool.

When closed, or "engaged", the movement of the apron moves relative to the rotation of the lead screw. This lead screw is precise in nature and can be described in terms of threads per inch (or per meter or millimeter in case of metric).

The lead screw is also connected somehow (there are various methods) to the main motor of the lathe and this ratio of gears can be changed so that many variations of threading, all relative to the thread of the lead screw, can be accomplished on the workpiece being turned/threaded by the screw cutting operation.

On a basic older style lathe, the screw begin cut can not be cut in a single pass of the cutting tool. This means that the half-nut may need to be disengaged to reverse the direction of the apron allowing for the cutting operation to be repeated, each time engaging and re-engaging the half nut, until the depth of the screw being cut has reached a final thread depth.